This invention relates generally to coaxial adaptors and more particularly to magnetron coaxial adaptors.
As is known in the art, a magnetron is a diode vacuum tube which has a magnetic field perpendicular to the electric field between a cathode and an anode electrode of the tube. A number of resonator cavities are disposed within the anode electrode in a cylindrical pattern around a cylindrically shaped cathode electrode. A permanent magnet is disposed around the anode electrode and cathode electrode to provide a magnetic field in a direction parallel to the axis of the tube. The resonant cavities generally have one of a number of cavity geometries as are known by ones of ordinary skill in the art, such as slotted, vaned or slotted-hole configurations.
In operation, power is provided to a heater disposed adjacent the cathode so that a surface of the cathode is raised to an elevated temperature to provide emission of electrons from the cathode surface. The emitted electrons are accelerated toward the anode electrode by the electrical field present between the anode and cathode electrodes. In some magnetron tubes, the cathode is directly heated whereas in other tubes a separate element is used to heat the cathode. The cathode is generally biased at a voltage potential less than that of the anode so that the negatively charged electrons are attracted toward the anode. The accelerated electrons, in passing through the constant and nearly uniform magnetic field are deflected such that energy is coupled to the resonator cavities within the anode block. The plurality of resonator cavities provides a periodic structure which oscillates at a frequency generally dependent on the geometry and dimensions of the cavity structures. The RF output power is extracted from one or more of the oscillating cavities generally by either a coaxial line output or a waveguide output.
For coaxial line outputs, a conductive cylindrical rod having a looped end portion is disposed within one of the cavities with a second end extending out of the magnetron housing. Coaxial line looped outputs are typically limited to lower power and lower frequency magnetron applications, since coaxial structures generally introduce greater insertion loss than waveguide structures. Moreover, in high power applications, there is a greater likelihood that voltage breakdown arcing will occur between the center and outer conductors.
On the other hand, waveguide outputs are typically used for higher power and higher frequency applications. For this type of output, power is extracted from a back wall of one of the cavities by means of a slit having dimensions which generally expand until the dimensions correspond to those of the output mating waveguide. Although a waveguide output generally provides a higher power handling and lower insertion loss transition for energy coupled out of the magnetron source, waveguide output magnetron sources for some applications may be undesirable because of their relative large size, bulk and cost.
Alternatively, a waveguide output magnetron may be provided by having the aforementioned coaxial line loop output disposed within a waveguide. In this configuration, the end of the cylindrical rod provides a probe which when inserted into a section of the waveguide converts the energy from a TEM mode to a waveguide mode, such as the TE.sub.10 mode. The length of the portion of the probe which extends into the waveguide is typically about one quarter wavelength at the frequency of interest. A shorting plate is disposed at one end of the waveguide at an appropriate distance from the probe for providing maximum transmission of the energy to the output end of the waveguide.
For many applications it would be desirable to have a high power and low cost magnetron source having a relatively low loss transition from a coaxial output probe of the magnetron to a standard coaxial connector.